Logging

Production Node services emit structured logs (JSON lines) so aggregators can query by fields — not unstructured `console.log` strings.

					logger.info({
  reqId: req.id,
  userId: req.user?.id,
  method: req.method,
  path: req.path,
  durationMs: Date.now() - start,
  status: res.statusCode,
}, 'request completed');
				

Levels: error/warn for actionable signals; info for request lifecycle; debug/trace gated by env. Redact passwords, tokens, PAN, and health payloads. Bind correlation/request ID at middleware and pass through async context (AsyncLocalStorage).

Volume control: sample debug logs; avoid logging full bodies; use metrics for high-cardinality counts.

On interviews: structured vs text; correlation across microservices; what never belongs in logs.

Common pitfalls: logging secrets on error paths; multiple logger formats; synchronous logging under load blocking the event loop (choose performant logger or async transport).

The trade-off is balancing simplicity, performance, safety, and operability — name which axis you optimized and what cost you accepted.

Checklist:

• JSON schema stable across services.
• Request ID in every log line for a trace.
• Redaction rules tested.
• Log level configurable per environment.